I'm no pro, but this worked for me on both my iphone 4 and ipad 2, I subscribed to imatch after upgrading to iOS6 and experienced some of the same issues reported on this thread. Here's what worked for me:
Go to "Settings>Music
Scroll down to where it says "Home Sharing",
If there is a box underneath your Apple ID with a field that says "Required", enter your password.
Once again, I'm no expert but it worked for me. Everything was there where I wanted it. Hopefully this will help others. No resets, no complicated steps, no third party software to download. Perfect.